fix: many2many association with duplicate belongs to elem (#6206)

callbacks/associations.go

@@ -51,25 +51,40 @@ func SaveBeforeAssociations(create bool) func(db *gorm.DB) {
 					}
 
 					elems := reflect.MakeSlice(reflect.SliceOf(fieldType), 0, 10)
+					distinctElems := reflect.MakeSlice(reflect.SliceOf(fieldType), 0, 10)
+					identityMap := map[string]bool{}
 					for i := 0; i < rValLen; i++ {
 						obj := db.Statement.ReflectValue.Index(i)
 						if reflect.Indirect(obj).Kind() != reflect.Struct {
 							break
 						}
-
 						if _, zero := rel.Field.ValueOf(db.Statement.Context, obj); !zero { // check belongs to relation value
 							rv := rel.Field.ReflectValueOf(db.Statement.Context, obj) // relation reflect value
+							if !isPtr {
+								rv = rv.Addr()
+							}
 							objs = append(objs, obj)
-							if isPtr {
-								elems = reflect.Append(elems, rv)
-							} else {
-								elems = reflect.Append(elems, rv.Addr())
+							elems = reflect.Append(elems, rv)
+
+							relPrimaryValues := make([]interface{}, 0, len(rel.FieldSchema.PrimaryFields))
+							for _, pf := range rel.FieldSchema.PrimaryFields {
+								if pfv, ok := pf.ValueOf(db.Statement.Context, rv); !ok {
+									relPrimaryValues = append(relPrimaryValues, pfv)
+								}
+							}
+							cacheKey := utils.ToStringKey(relPrimaryValues...)
+							if len(relPrimaryValues) != len(rel.FieldSchema.PrimaryFields) || !identityMap[cacheKey] {
+								if cacheKey != "" { // has primary fields
+									identityMap[cacheKey] = true
+								}
+
+								distinctElems = reflect.Append(distinctElems, rv)
 							}
 						}
 					}
 
 					if elems.Len() > 0 {
-						if saveAssociations(db, rel, elems, selectColumns, restricted, nil) == nil {
+						if saveAssociations(db, rel, distinctElems, selectColumns, restricted, nil) == nil {
 							for i := 0; i < elems.Len(); i++ {
 								setupReferences(objs[i], elems.Index(i))
 							}

tests/associations_many2many_test.go

@@ -393,3 +393,33 @@ func TestConcurrentMany2ManyAssociation(t *testing.T) {
 	AssertEqual(t, err, nil)
 	AssertAssociationCount(t, find, "Languages", int64(count), "after concurrent append")
 }
+
+func TestMany2ManyDuplicateBelongsToAssociation(t *testing.T) {
+	user1 := User{Name: "TestMany2ManyDuplicateBelongsToAssociation-1", Friends: []*User{
+		{Name: "TestMany2ManyDuplicateBelongsToAssociation-friend-1", Company: Company{
+			ID:   1,
+			Name: "Test-company-1",
+		}},
+	}}
+
+	user2 := User{Name: "TestMany2ManyDuplicateBelongsToAssociation-2", Friends: []*User{
+		{Name: "TestMany2ManyDuplicateBelongsToAssociation-friend-2", Company: Company{
+			ID:   1,
+			Name: "Test-company-1",
+		}},
+	}}
+	users := []*User{&user1, &user2}
+	var err error
+	err = DB.Session(&gorm.Session{FullSaveAssociations: true}).Save(users).Error
+	AssertEqual(t, nil, err)
+
+	var findUser1 User
+	err = DB.Preload("Friends.Company").Where("id = ?", user1.ID).First(&findUser1).Error
+	AssertEqual(t, nil, err)
+	AssertEqual(t, user1, findUser1)
+
+	var findUser2 User
+	err = DB.Preload("Friends.Company").Where("id = ?", user2.ID).First(&findUser2).Error
+	AssertEqual(t, nil, err)
+	AssertEqual(t, user2, findUser2)
+}